This invention relates to magnetic field detection and in particular to fibre optic magnetic field sensors and their use in magnetic gradient detection.
Interferometric fibre optic magnetometers are already known for example from "A fibre-optic dc magnetometer" by K. P. Koo et al. Journal of Lightwave Technology, Vol. LT-1, No. 3, September 1983, P 524-5. They basically comprise an all-fibre Mach-Zehnder interferometer, one arm of which is magnetically sensitised by, for example, being bonded to a strip of magnetostrictive material or coated with a magnetostrictive material. A d.c. magnetic field has the effect of altering the optical path length of the sensitised arm and the interferometer output which is proportional to the differential path length change (.DELTA.L) is thus related to the magnetic field (H), .DELTA.L.alpha.H.sup.2. To overcome inaccuracies due to noise etc, an ac bias field at frequency w is applied to the sensitised arm and then the interferometer output at w is directly proportional to the dc magnetic field.
A fibre-optic magnetic gradient detector is described by K. P. Koo et al in the article on pages 509-513 of the Journal of Lightwave Technology, Vol. LT-1, No. 3, September 1983. In this case both arms of the all-fibre Mach-Zehnder interferometer are magnetically sensitised and ac bias fields at the same frequency are applied to both sensitised portions. The detector described in this article only measures the magnetic gradient in one direction, that is between the two arms.
In our co-pending G.B. Application No. 8504729 published under (Ser. No. 2171514A) (p. Extance-R. E. Jones 17-17) there is described a fibre optic magnetic gradient detector which detects two orthogonal magnetic gradient components simultaneously and employs three sensitised optical fibre portions, two in one arm and one in the other arm of an all-fibre Mach-Zehnder interferometer, and ac bias fields at two different frequencies. The sensitised optical fibre portions are wound in the form of planar circular sensing coils, the single mode optical fibre employed being magnetically sensitised by, for example, being coated with a magnetostrictive material.